pgsql: Pass BackgroundWorker entry in the parameter file in EXEC_BACKEN

Pass BackgroundWorker entry in the parameter file in EXEC_BACKEND mode

The BackgroundWorker struct is now passed the same way as the Port
struct. Seems more consistent.

This makes it possible to move InitProcess later in SubPostmasterMain
(in next commit), as we no longer need to access shared memory to read
background worker entry.

Reviewed-by: Tristan Partin, Andres Freund, Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/388491f1e5e63fe97c7cca26d18b64321973d423

Modified Files
--------------
src/backend/postmaster/bgworker.c           |  23 +-----
src/backend/postmaster/postmaster.c         | 120 +++++++++++++++++-----------
src/include/postmaster/bgworker_internals.h |   4 -
3 files changed, 75 insertions(+), 72 deletions(-)